Photomultiplier tube having impact ionization diode collector

ABSTRACT

A photomultiplier tube (PMT) converts a received light signal to an output electrical signal of substantially greater intensity by employing a photocathode to convert incident light to free electrons, a plural dynode accelerating structure for effectively multiplying said free electrons, and an impact ionization diode (IID) for further multiplying and collecting said free electrons to provide a corresponding electrical output signal. The PMT can be an electrostatic device, in which the photocathode and the dynodes are mounted in opposed staggered positions, or a static crossed field device, in which the photocathode and the dynodes all are mounted opposite an accelerating rail and a magnetic field is provided to urge the electrons laterally along the tube. The IID&#39;&#39;s junction is reverse biased and the entire diode is maintained at a substantially higher potential than the last dynode. The PMT can be gain controlled or turned off without affecting dynode potentials by controlling the IID&#39;&#39;s potential. Due to the gain provided by the IID, dynode current can be reduced greatly, thereby to increase substantially the tube&#39;&#39;s life without affecting its overall gain.

[451 May 20, 1975 United States Patent 1 Goehner PHOTOMULTIPLIER TUBEHAVING IMPACT IONIZATION DIODE COLLECTOR [75] Inventor: Ronald H.Goehner, Wayne, NJ.

[73] Assignee: Varian Associates, Palo Alto, Calif.

[22] Filed: July 11, 1974 21 Appl. No.: 437,704

Primary ExaminerRohert Segal Attorney, Agent, or Firm-Stanley Z. Cole;D. R. Pressman; Robert K.. Stoddard {57] ABSTRACT A photomultiplier tube(PMT) converts a received -600 WRT AP 6500 -600 W RT AP light signal toan output electrical signal of substantially greater intensity byemploying a photocathode to convert incident light to free electrons, aplural dy node accelerating structure for effectively multiplying saidfree electrons, and an impact ionization diode ("0) for furthermultiplying and collecting said free electrons to provide acorresponding electrical output signal. The PMT can be an electrostaticdevice, in which the photocathode and the dynodes are mounted in opposedstaggered positions, or a static crossed field device, in which thephotocathode and the dynodes all are mounted opposite an acceleratingrail and a magnetic field is provided to urge the electrons laterallyalong the tube. The llDs junction is reverse biased and the entire diodeis maintained at a substan tially higher potential than the last dynode.The PMT can be gain controlled or turned off without affecting dynodepotentials by controlling the llDs potential. Due to the gain providedby the D, dynode current can be reduced greatly, thereby to increasesubstantially the tubes life without affecting its overall gain.

8 Claims, 7 Drawing Figures ACCELERATING PLATE 24 PATENTED MAY 2 01975SHEET 10F 2 2 H2 n E: E a E;

8% E 8 o8- o8- 8? PATENIED HAYZO I975 $885.17 8

sum 2 or 2 1 PHOTOMULTIPLIER TUBE HAVING IMPACT IONIZATION DIODECOLLECTOR FIELD OF INVENTION This invention relates to photomultipliertubes in general and in particular to photomultiplier tubes whose gaincan be controlled and increased and whose life can also be increased.

PRIOR ART Heretofore photomultiplier tubes consisted of a vacuumenclosure containing a photocathode, a series of dynodes, and anelectron collector. Light admitted through a window in the tube andshined on the photocathode caused electrons to be emitted therefrom.These electrons were made to impinge on the successive dynodes, causingmultiple electron multiplications by secondary emission. Afterimpingement on the last dynode, the electrons were collected anddelivered on an output lead of the tube to provide an output signalwhich was proportional to, but much stronger than, the input lightsignal. Various means were employed to cause te electrons to impinge onthe successive dynodes, including radio frequency fields, magneticfields, successively increasing potentials applied to the dynodes, andstrategic placement of the dynodes; as these means are either wellknownin the art or de scribed in the other applications referred to herein,and as they are not directly germane to the present invention, they willnot be discussed in detail herein.

One major difficulty associated with prior art high speed photomultipiertubes is poor longevity. Such tubes have short longevity because theelectron stream therein impinges on the dynodes with sufficient force towear down the dynode surfaces to such an extent as to render theiroperation marginal, usually after only 100 hours of operation. Moreoverdue to electron bombardment of the dynodes, material is liberated fromthe dynodes which poisons the atmosphere in the tube (which contains acritical partial pressure of cesium), destroying the properties of thephotocathode therein. This low longevity is quite serious since highspeed photomultiplier tubes of the type described herein sell at pricesfrom $6,000 to $l0,000 each; when coupled with their extremely shortlife, the cost per hour of operating these tubes becomes extremely high.

Another disadvantage of high speed photomultiplier tubes of the typedescribed herein is lack of gain control capability. This is because, tooperate properly, the various electrodes of the tube must haverelatively critical potentials applied thereto. While changing thesepotentials will tend to change the gain of the tube, it is not possibleto do this in a practical sense because changing the potentials of theelectrodes will usually adversely effect the operation of the tube tosuch an extent as to render it inoperable for the intended purpose.

Accordingly several objects of the present invention are to increase thelife of high speed photomultiplier tubes and to provide a means of gaincontrolling photomultiplier tubes. Further objects and advantages of theinvention are to increase the speed of photomultiplier tubes, to reducethe dynode current in photomultiplier tubes, to decrease the cost perhour of operation of photomultiplier tubes, to provide a means ofreadily turning off high speed photomultiplier tubes, to provide a meansof increasing gain of photomultiplier tubes, and to provide a new typeof hybrid photomultiplier tube. Further objects and advantages of theinvention will become apparent from a consideration of the ensuingdescription thereof.

REFERENCES Additional details of tubes of the types described herein aredescribed in the following related cases which are assigned to thepresent Applicants assigneed and which are incorporated herein byreference:

U.S. Pat. No. 3,757,l57; I971 Nov. 26, Abraham- /Enck: Improved dynodestructure for crossed field electron multiplier devices.

Application Ser. No. 452,151; 1974 Mar. 18; BuckfGoehner/Guy: Means toreduce dark current in photomultipliers.

Application Ser. No 471,418; 1974 May 20; Enck: Means to reduce darkcurrent in photomultipliers.

Additional details of diodes of the type described herein are describedin, e.g., Theoretical Optimization of EPS Targets by Silzars, Knight andNorris, ED-2O IEEE Trans. Elec. Dev. 193 (March 1974).

REFERENCE NUMERALS l0 enclosure 38 contact to substrate 34 I2 window 40contact to surface layer 36 I4 light 42 insulating layer 16 photocathode44 chip capacitor I8 electrons 46 diode bias 20 first dynode (FIG. I) 48first slat 22 second dynode (FIG. 2) 50 second slat 24 acceleratingplate 52 third slat 25 ceramic ring 54 magnetic field 26 impactionization diode 56 rail 28 center conductor 58 first dynode (FIG. 4) 30outer conductor 60 second dynode (FIG. 4) 32 output connector 62 meshwindow 34 substrate of IID26 64 aperture 36 surface layer of IID26DRAWINGS FIG. 1 IS a cross-sectional view of a high speedphotomultiplier tube according to the invention.

FIGS. 2A and 2B are cross-sectional and plan views of the diodecollector portion of the tube of FIG. 1.

FIG. 2C is an equivalent circuit diagram of the diode collector portionof FIG. 1.

FIGS.

FIGS. and 3B are cross-sectional and plan views of the diode per se ofFIG. 2A.

FIG. 4 is a cross-sectional view of the dynode and collector portion ofa crossed field photomultiplier tube according to the invention.

DESCRIPTIONFIG. l-ELECTROSTATIC TUBE The electrostatic tube of FIG. 1comprises a vacuum enclosure 10 containing a window 12 for admission oflight 14 so that said light can be directed to a photocathode l6.

Photocathode 16 can be a III-V device, such as binary GaAs, which has a053p. sensitivity, or a quaternary Ill-V compound, such as lnGaAsP,which has a 1.06pm sensitivity. Alternatively S-l (l.06u) or 5-20 (053p)photocathodes can be used.

Electrons l8 generated by photocathode 16 are directed to a first dynode20 and thence to a second dynode 22. From the second dynode 22 theelectrons pass through an aperture in an accelerating plate 24 to animpact ionization diode 26 where they are collected and delivered on acoaxial line comprising a center conductor 28 and an outer conductor 30.An electrical output signal is taken at output connector 32 at the endof the coaxial line.

Enclosure is fabricated of a series of flange sections, such as thatforming accelerating plate 24, inter sperced with cylindrical ceramicsections, such as 25. The various electrodes in the tube are connectedto or are integral with the flanges, as described in more detail infra.

Impact ionization diode 26, best shown in FIGS. 3A and 3B, comprises asemiconductor substrate 34 of one conductivity type having a raised ormesa portion across which is formed an epitaxial thin surface layer 36of the same conductivity type. A contact 38 is made to substrate 34 anda Schottky barrier contact 40 is made to surface layer 36. As best shownin FIG. 38, contact 40 has a broad circular lefthand (Schottky barrier)portion connected to a rectangular right-hand (contact) portion by anarrowed neck portion. An insulating layer of silicon dioxide 42 (FIG.3A) is provided to insulate neck portion of contact 40 from substrate34.

The entire impact ionization diode 26 is about l0 mils by 25 mils insize, with the mesa portion thereof being about mils in size. Substrate34 is preferably P- type silicon having a resistivity of ().l ohm-cm andP- type surface layer 36 is formed epitaxially to have a re sistivity ofabout ohmcm. The Schottky barrier portion of contact 40 preferably isaluminum about 400 A thick.

Impact ionization diode 26 is mounted on a chip-type bypass capacitor44. Capacitor 44 is mounted between substrate 34 of diode 26 and outerconductor 30 of the coaxial line, thereby to provide an ac connection orbypass between the anode 34 of capacitor 26 and outer conductor 30, asindicated in the equivalent circuit diagram of FIG. 2C. Diode 26 isback-biased by a negative source 46 connected to its anode, the cathodeof diode 26 being connected to center conductor 28 which is normallyheld at ground potential, as is outer conductor 30.

Various other potentials are applied to the tube electrodes, asindicated in FIG. 1. Accelerating plate 24 is maintained at a potentialof -3,000 to 6,5()() volts with respect to diode 26 and ground, theprecise poten tial being adjusted according to the amount of gaindesired in the tube. Dynode 22 is maintained at a potential of 60()volts with respect to accelerating plate 24, dynode is maintained at apotential of 1,200 volts with respect to accelerating plate 24 andphotocathode 16 is maintained at a potential of l ,800 volts withrespect to accelerating plate 24.

In order to provide high initial accelerating force adjacent eachelectron emissive surface, thereby to reduce electron transit timedisplacement, a series of slats" or projections into the main cavity ofthe tube are provided. Each slat is normally maintained at a higherpotential than either the electron emissive surface downstream orupstream thereof, thereby to shape the electron accelerating fields inthe tube properly. Due to electron ballistic effects, substantially noelectrons are intercepted by such slats. A first slat 48 projects intothe space between photocathode l6 and first dynode 20 and is maintainedat a potential of 600 volts with respect to accelerating plate 24; asecond slat 50 is positioned in the space between first dynode 20 andsecond dynode 22 and is normally maintained at the potential ofaccelerating plate 24; and a third slat 52 is positioned between seconddynode 22 and accelerating plate 24 and is normally maintained at apotential of 600 volts with respect to accelerating plate 24. Due to theflange-type section construction of tube 10, each slat may be formed byappropriately shaping each of the flanges forming part of the structureof the tube.

The potentials of the photocathode, dynodes, and slats are varied(preferably by simple voltage divider circuitry not shown) to providethe aforementioned fixed potential differences between these electrodesand plate 24 when the potential of plate 24 is varied.

FIG. 4-STATIC AND DYNAMIC CROSSED FIELD TUBE EMBODIMENTS In theelectrostatic tube of FIG. 1, the progression of the electrons from thephotocathode to the first dynode, from dynode-to-dynode, and from finaldynode to impact ionization diode collector 26 was achieved through theuse of strategic placement of electrodes maintained at successivelyhigher potentials. In the crossed field tube of FIG. 4, all of thedynodes are maintained at increasing potentials and are mounted atsuccessively elevated positions, but since they are all mounted on oneside of the tube, additional means are required to cause the electronstream to impinge upon these dynodes and to be drawn from the dynodes.In a static crossed field tube, an orthogonal magnetic field 54 and anelevated potential rail 56 are provided for this purpose. In a dynamiccrossed field photomultiplier (DCFP), an RF (radio frequency) field isalso pro vided in the tubes cavity, as indicated. Full details ofdynamic and static crossed field tubes, including the operation,envelope and other physical structure thereof are described in theaforementioned AbrahamlEnck US. Pat. No. 3,757,157.

The tube contains a photocathode 16 as in FIG. I positioned under awindow in the envelope of the tube so as to receive light 14 from asource outside the tube, a first dynode 58 positioned to receiveelectrons from photocathode l6, and a second dynode structure 60 havinga mesh opening thereof 62 designed to admit electrons from dynode D1 toan impact ionization diode collector 26 similar to that of FIG. I. Inthe static tube, potentials are applied to the dynodes and collectorstructures as indicated. As in FIG. I, the potentials of photocathode l6first dynode 58 and rail 56 are always maintained at the same potentialwith respect to second dynode 62 (D2), whose potential can be variedwithin the range indicated to control the gain of the tube. In the DCFP,the dynodes preferably are all at the same potential and physicalheight, or can be a single elongated dynode, as indicated in saidAbraham/Enck patent.

Magnetic shielding means (not shown) are also provided around diode 26and the area thereabove to shield the electron stream in the area abovethe diode from magnetic field 54 so that the electron stream will not becurved in the region immediately above the diode and so that the diodewill be shielded from the magnetic field which might interfere withoperation thereof.

OPERATION The operation of both type tubes is similar but will bedescribed with respect to the presently preferred em bodirnent (FIG. 1).When a light source 14 (e.g., from a distant laser of the correctfrequency is directed through window 12 on photocathode 16, it causesphotocathode 16 to emit electrons in well-known fashion. The electronsemitted by photocathode 16 are initially accelerated by first slat 48and then by dynode 20 upon which they impinge violently to generatecopious secondary electrons in well-known fashion. The secondaryelectrons from dynode 20 are initially accelerated by second slat 50 andthen by second dynode 22 upon which they impinge more violently, againto generate copious secondary electrons. These are initially acceleratedby third slat 52 and then by accelerating plate 24 such that they passthrough aperture 64 in the accelerating plate impinge even moreviolently upon impact ionization diode 26. The electrons are collectedby diode 26, causing an output signal to be provided on center conductor28 with respect to outer conductor 30. This output signal will vary inaccordance with the intensity of input light signal 14, but will havesubstantially more energy than the former.

Due to the gain provided in diode 26i.e., an electron multiplication ofabout l,O00the current on the dynodes can be (and in the describedembodiment is) substantially reduced while still providing the sameoverall gain. This reduces dynode wear and outgasing of the dynodes dueto electron impingement thereon, thereby increasing the longevity of thetube. The current on the dynodes can actually be reduced to about 1microampere, a level low enough to reduce dynode wear sufficiently sothat the operating life of the tube will approximate its shelf life.This contrasts with prior art photomultipliers in which the tubeperformance was degraded to about 90 percent of its original quality inabout 100 hours.

An additional and substantial advantage of the invention is the gaincontrol available by varying the potential between the collector (diode26) and the dynode structure. As is well-known, it is impractical toreduce the gain of a conventional photomultiplier tube by reducing theinterdynode potentials because these are usually set to operate atcritical values so that changing same will disrupt the operation of thetube sufficiently to preclude this as a practical method of changinggain. In the present tubes, by varying the potential of the acceleratingplate 24 with respect to ground or the potential of collector diode 26,and by varying the potentials of all other dynodes and the photocathodeso as to maintain a constant potential between these elements and theaccelerating plate, it is possible to decrease the gain of the tube by asubstantial factor, or even to turn it off entirely.

That is, when the potential of accelerating plate is at its maximumnegative value (6,500 V) the gain of the tube will be maximized, butwhen the potential is at its least negative value and range indicated(3,000 V) the gain of the tube will be minimized.

Through the use of this method of varying the potentials on the tube,the tube can be automatically gain controlled (AGCd) as is possible withother amplification devices such as transistors and vacuum tubes. Thisis believed to represent a substantial advance in the art.

The tube can also be turned off by reducing the potential betweenaccelerating plate and diode 26 to a low value.

Also the gain of the tube can be increased with respect to prior arttubes merely by incorporating impact ionization diode 26 therein withoutreducing dynode current. Moreover the gain and life of prior art tubescan both be increased by incorporating diode 26 and partially reducingdynode current.

While the above description contains many specificities, these shouldnot be construed as limitations on the scope of the invention.Accordingly the scope of the invention should be construed onlyaccording to the following claims and their legal equivalents.

What is claimed is:

1. A photomultiplier tube comprising a hermetically sealable enclosurehaving a transparent area for admission of light, a photocathodepositioned in said enclosure for receiving said light and convertingsame to free electrons, at least one dynode positioned in said enclosurefor receiving said free electrons from said photocathode and multiplyingsame, and means positioned to receive free electrons from said dynodefor simultaneously providing multiplication of received electrons andcollection of same on a conductor to provide an output signal on saidconductor, said means positioned to receive free electrons comprising adiode capable of multiplying and collecting received free electrons byimpact ionization.

2. The tube of claim 1 wherein said dynode is positioned facing saidphotocathode and said means is positioned facing said dynode.

3. The tube of claim 1 including a plurality of dynodes, each maintainedat a greater potential than the one next closest to said photocathode.

4. The tube of claim 1 further including means for providing a magneticfield through said tube for urging free electrons emitted by saidphotocathode to said dynode, said tube including an accelerating rail,said photocathode and said dynode facing said accelerating rail.

5. The tube of claim 1 wherein said enclosure is evacuated and furtherincluding means for maintaining said dynode and said means atsuccessively greater potentials than said photocathode, and means forcontrolling independently the potential of said means.

6. The tube of claim 1 including a plurality of dynodes, each maintainedat a greater potential than the one next closest to said photocathode,adjacent ones of said dynodes, said photocathode, and said means facingeach other, said means being maintained at a greater potential than anyof said dynodes.

7. The tube of claim 1 including a plurality of dynodes, each maintainedat a greater potential than the one next closest to said photocathode,an accelerating rail facing all of said dynodes and said photocathode,said means being maintained at a greater potential than any of saiddynodes, and means providing a magnetic field through said dynodes in adirection for urging electrons emitted by said photocathode along saiddynodes to said means.

8. The tube of claim 4 further including means for injecting radiofrequency energy into said enclosure for urging said electrons toimpinge upon said dynode.

1. A photomultiplier tube comprising a hermetically sealable enclosurehaving a transparent area for admission of light, a photocathodepositioned in said enclosure for receiving said light and convertingsame to free electrons, at least one dynode positioned in said enclosurefor receiving said free electrons from said photocathode and multiplyingsame, and means positioned to receive free electrons from said dynodefor simultaneously providing multiplication of received electrons andcollection of same on a conductor to provide an output signal on saidconductor, said means positioned to receive free electrons comprising adiode capable of multiplying and collecting received free electrons byimpact ionization.
 2. The tube of claim 1 wherein said dynode ispositioned facing said photocathode and said means is positioned facingsaid dynode.
 3. The tube of claim 1 including a plurality of dynodes,each maintained at a greater potential than the one next closest to saIdphotocathode.
 4. The tube of claim 1 further including means forproviding a magnetic field through said tube for urging free electronsemitted by said photocathode to said dynode, said tube including anaccelerating rail, said photocathode and said dynode facing saidaccelerating rail.
 5. The tube of claim 1 wherein said enclosure isevacuated and further including means for maintaining said dynode andsaid means at successively greater potentials than said photocathode,and means for controlling independently the potential of said means. 6.The tube of claim 1 including a plurality of dynodes, each maintained ata greater potential than the one next closest to said photocathode,adjacent ones of said dynodes, said photocathode, and said means facingeach other, said means being maintained at a greater potential than anyof said dynodes.
 7. The tube of claim 1 including a plurality ofdynodes, each maintained at a greater potential than the one nextclosest to said photocathode, an accelerating rail facing all of saiddynodes and said photocathode, said means being maintained at a greaterpotential than any of said dynodes, and means providing a magnetic fieldthrough said dynodes in a direction for urging electrons emitted by saidphotocathode along said dynodes to said means.
 8. The tube of claim 4further including means for injecting radio frequency energy into saidenclosure for urging said electrons to impinge upon said dynode.